Babies wake at any hour, teenagers refuse to get out of bed, older adults find a good night’s rest harder to achieve. Why do our sleep patterns shift with age?
Between dozing off and the alarm shrieking, our bodies and brains undergo quite a few changes. Sleep researchers call the first stage of sleep N1. It is light, and if woken from it we might not realise we had drifted off in the first place. Our night-time voyage is well established by the time we enter the second stage of sleep, N2. This can be identified in the laboratory by distinctive brainwaves such as sleep spindles, involving a sudden burst of brain activity. N3 is referred to as deep sleep or slow-wave sleep because of the brain activity, which is more comparable to rolling waves. Eventually, we reach rapid eye movement (REM) sleep – a stage characterised by brain activity not unlike that seen during our waking lives. Vivid dreams and bodily paralysis also occur – perhaps the latter keeps us safe, by preventing us from acting out the former. Adult sleep cycles take about 90 minutes.
Two main processes control when we sleep. The first is called sleep homeostasis and refers to our sleep drive. The longer we have been awake, the greater our pressure to fall asleep. This alone can’t explain why we might struggle to sleep during the day, despite having been up all night; or find ourselves nodding off in the evening even though we snuck a daytime nap. Instead, the circadian process helps to explain such occurrences. This clock-like mechanism is controlled by a central pacemaker in the brain. Our internal body clocks often run at slightly over 24 hours a day, so we use zeitgebers (time-givers) from the environment, to tweak our internal clocks so as to fit with the world outside. Cues come from all around – including the time we eat or exercise – but none is as important as light exposure. Light helps our brain know it is time to be awake and darkness allows us to produce melatonin, giving our bodies a sign that it is time to sleep. Without cues from the environment, bedtime would typically become later night-on-night, and soon we’d find ourselves ready to sleep when we should be waking up.